At some point during the growth of an insect larva, a gene called Dll switches on and helps organize some of its cells into legs. If for some reason Dll is shut off, the insect will produce only stumps. In the early 1990s scientists were surprised to discover that almost identical copies of this gene can be found in mammals and other vertebrates—and that they too switch on as legs form. This was surprising for two reasons. For one thing, insects and vertebrates have radically different limbs: ours have bone inside and muscle outside, while bugs are the reverse—their flesh is protected by an armored exoskeleton. For another thing, insects and vertebrates are only distantly related: our last common ancestor lived perhaps a billion years ago and was assumed to be limbless, like a flatworm. Researchers therefore imagined that the two lineages evolved their limbs—and the genes that build them—independently.
The Dll discoveries inspired a group of biologists led by Grace Panganiban of the University of Wisconsin and Steven Irvine of the University of Chicago to survey some of the other animals that also descended from that billion-year-old ancestor. They looked at odd little creatures called velvet worms, which walk around on forest floors with fleshy pads. They looked at sea urchins, which are studded with little tubes that they use to roll around the ocean floor. They looked at sea squirts, which use a big, footlike projection to clamp onto coral reefs. Remarkably, as the researchers announced last May, Dll-like genes become active in the developing appendages of every animal they looked at.
Data pouring out of other labs suggest that all these animals are similar in other ways as well. Insects and people, for instance, also have very different eyes—ours focus light through a pupil onto a retina, while fly eyes are made of hundreds of honeycomb-shaped image sensors. Researchers had again figured that both groups evolved eyes on their own. But over the past few years several labs have shown that a number of key genes that produce vertebrate and insect eyes are almost identical—and last March it was reported that even squid use the same genetic equipment. Recent studies suggest that the development of certain features—head-to-tail body axis, back-to-belly orientation, and even the heart—is guided by the same basic genes in a wide variety of animals.
At the very least, these results suggest that our billion-year-old ancestor already had those genes. Exactly what that ancestor looked like is a trickier question, though. Perhaps instead of limbs it had some other projections from its body wall, such as stalks alongside its mouth for feeding. Rather than an eye, it might have had a light-sensitive patch of cells on its skin. And rather than a heart, it might have had a confluence of blood vessels. Only later did animals along different evolutionary branches use these genes to produce different kinds of limbs, hearts, and eyes. We have this huge jigsaw puzzle and only a few pieces on the table right now, but the last common ancestor of insects and humans may have been more complicated than we previously thought, says Panganiban. We may be able to draw a picture of the common ancestor pretty soon, and it may not look like a flatworm.
